Protection of Centrifugal Compressor system with cooling loop

[Bill3752]

I am a bit stumped about handling this system. See attached. At blocked in relief (900 psig), the flow to the system will cease, but before that happens the loop pressure will increase substantially. This will cause the suction pressure to the compressor to increase, ultimately causing the RV to lift. Under normal conditions about 90% of the flow is recycled through the cooler.

I suspect I am missing something simple. Any help would be appreciated.

 

[LittleInch]

Under normal conditions about 90% is recycled? What? Makes little sense.

So why would another 10% make any difference?

There is no reason why the inlet pressure would increase.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Sounds normal to me what you describe. So in normal operation, you've got most of the compressor flow going through recycle, and only 10% is exported.
What are you trying to resolve here : "I am a bit stumped about handling this system"
Are you trying to minimize the frequency of RV activation ? If so, are there instrumented shutdown trip loops in this circuit ? - No trip loops shown on sketch.
On the other hand, is it a controls issue : is the inlet PCV acting too slow when there is a sudden spike in exit pressure, causing the exit RV to lift ? I dont see any check valves on the compressor exit line either that should prevent reverse flow into circuit from export line - sketch is too brief.

 

[Bill3752]

Appreciate your comments...

Entire system, including the suction drum, is designed for 900 psig. George, I am trying to determine if the system can be over pressured (and ultimately the relief rate). So at relief the flow to the system will stop since the upstream pressure < 900 psig. However, we are assuming that the compressor will still continue to operate. So it seems that the recycled gas will continue to increase in pressure (assuming that the suction pressure to the compressor increases) and temperature. I am checking my sanity on this. If what I just proffered is true, then does this turn into a more complicated hydraulic model case that is a function of time? Or am I missing something simple?

Kindest Regards

 

[georgeverghese]

The blocked in relief scenario on centrifugal compressors is typically as follows:
a)Compressor discharge is completely closed, and so are internal recycle lines
b)Compressor continues to run at max permissible speed which is possibly 105% of max operating - speed alarm high setting
c)Suction pressure is at PSHH, and feed gas is at its highest density
d)Export line PSV is operating at relieving pressure - 110% of SP. Neglect any frictional losses at compressor cooler if you want to be conservative.
e)Feed gas line is fully open

Look up the polytropic head vs actual flow curve for this machine at this speed and work out where the compressor operating point is. That will give you flow. Check that the corresponding power demand at this point is within compressor driver power delivery with no fouling losses (if there is a GT as driver).

At (d), in some cases, you may need to work out the operating point at 100% of set pressure.

 

[Bill3752]

George, Thanks for the very useful approach. Could you clarify what you define as the "internal recycle lines" in (a)? If this refers to the recycle line through the cooler back to the suction KO drum, then I believe I will run into a complication. Since the compressor is sized 10X the process flow, the compressor rate will be substantially higher than the available upstream flow; this is in part the cause of my confusion - i.e. does my available feed flow limit what can be put through the compressor, and by extension the design of the RV?

Or by internal, perhaps you are referring to recycle specific to the compressor itself?

 

[georgeverghese]

Yes, the recycle line with cooler would be closed at the time of relief. Controls behaviour cannot be relied upon during transient events as there is no assurance of response sufficient to address the transient.
You say there are limitations in feed flow. In any case, the process recycle loop is NOT a safety loop.
We can also deduce that if the upstream feedline PCV were to go through some transient, the recycle PCV may go wide open in its response.

 

[georgeverghese]

To "disable" part of the compressor throughput for this case, you could install a restriction orifice on a bypass across the recycle valve. And size the orifice for say 80% of compressor throughput that is seen at the time of relief( ie. PSV operating at 110% of SP, with compressor suction at PSHH, gas feed mol. wt at its highest). So the export PSV need only cater for 20% of this total throughput.
Check what export throughput you will then get at normal operating conditions (ie normal suction and discharge pressure), and see if this is acceptable to you and to Operations.
Obviously, there should be no block valve on this bypass line with RO. Add a cautionary note on the P&I D that this recycle valve bypass RO is not to be removed without Engineering Dept approval. And same for the Plant Ops Manual.
Make a detailed assessment of the upstream and downstream pressure at this RO at the time of relief for the sizing case.
Can you confirm there is a PSHH on compressor discharge
a) that trips the compressor
b)there is a check valve upstream of the export line block valve
c)the export line block valve goes auto close on compressor trip
d)the suction feed to this compressor process design pressure is less than 900psig, and has a PSV set <900psig.

 

[pierreick]

Hi,
Some schematics about surge control system
Pierre